Cuvette carrier

ABSTRACT

A cuvette carrier comprising: a plurality of walls defining a holding volume for a cuvette; a first and second transmissive region included in the plurality of walls; and a first optical polariser arranged to polarise light passing through the first transmissive region.

REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-part of U.S. application Ser. No. 15/750,971, filed Feb. 7, 2018, which is a national stage application under 35 USC 371 of International Application No. PCT/GB2016/052924 filed Sep. 20, 2016, which claims the benefit of GB Application No. 1516851.1 filed Sep. 23, 2015, the entire contents of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a carrier for holding sample cuvettes, in particular to a cuvette carrier having an optically polarising component.

BACKGROUND OF THE INVENTION

Dynamic light scattering (DLS) is a powerful technique for measuring the size of particles, for example nanomaterials and nanoparticulates. Typically, a suspension or solution containing the particles is placed in a transparent sample cuvette and is illuminated with a vertically polarised laser beam. Light scattered from the particles along a particular direction is detected in a plane orthogonal to the polarisation of the incoming light, and analysed using well known methods to determine properties of the particles. Light that is scattered in any direction from the particles may be detected, such as forward-scattered light, back scattered light and/or side-scattered light (e.g. scattered light at an angle of 90° to the incoming light).

A further development of dynamic light scattering, depolarised dynamic light scattering (DDLS), seeks to quantify the anisotropy in scattering from particles that deviate from a sphere of uniform refractive index, such as rods, ellipsoids, discs, or spherical Janus particles. In DDLS, a particular polarisation of scattered light is detected. For example, a sample may be illuminated with vertically polarised light, and the correlation function of the scattered light detected in each scattered polarisation, separately, along the detection path.

There are many laboratory instruments available that can perform dynamic light scattering, such as the Malvern Zetasizer® range. However, these instruments may not be able to measure different polarisation states of the scattered light, and so cannot perform depolarised dynamic light scattering measurements. Upgrading system hardware, or replacing a system altogether in order to perform DDLS may be undesirably expensive.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a cuvette carrier comprising a plurality of walls defining a holding volume for a cuvette; a first and second transmissive region included in the plurality of walls; and a first optical polariser arranged to polarise light passing through the first transmissive region.

The term “polariser” as used herein refers to a filter that passes a component of light with a specific polarisation, and blocks (at least partially) components of light with other polarisations. The term “polarise” refers to filtering light with a polariser so as to preferentially pass components of light with a specific polarisation, while at least partially blocking components of light with other polarisations.

The second transmissive region may be for illuminating a region within the holding volume, and the first transmissive region may be for detecting scattered light from the said region.

Such a cuvette carrier may be used to retrofit existing equipment to allow DDLS measurements to be performed using the optical polariser to detect the polarisation of the scattered light. In particular, the cuvette carrier may fit into the cuvette holder of an existing DLS measurement system. A cuvette containing a sample may then be put into the cuvette carrier.

In some embodiments, the cuvette carrier may additionally comprise a second optical polariser arranged to polarise light passing through the second transmissive region. An axis of polarisation of the first optical polariser may be orthogonal to an axis of polarisation of the second optical polariser, or may be parallel to an axis of polarisation of the second optical polariser.

The second optical polariser may be used, for example, to polarise light entering a cuvette in the cuvette carrier, before the light is scattered by a sample in the cuvette. Alternatively the second optical polariser may be used in conjunction with the first polariser to detect the polarisation of light scattered by the sample.

The first transmissive region and second transmissive region may be located in a first wall of the plurality of walls. This may be particularly useful when the first polariser is used to detect the polarisation of backscattered light from a sample, and the second polariser used to polarise the light beam entering the sample.

In some embodiments, the cuvette carrier may further comprise a third transmissive region located in a second wall of the plurality of walls; and a third optical polariser arranged to polarise light passing through the third transmissive region. In some embodiments, an axis of polarisation of the third optical polariser may be orthogonal to an axis of polarisation of the first optical polariser, or may be parallel to an axis of polarisation of the first optical polariser. The third optical polariser may be used, for example, to polarise side-scattered light scattered by a sample that has been illuminated through the second transmissive region. A detector, for example a detector in a DLS instrument, may then detect the polarised side-scattered light as part of a DDLS experiment.

Additionally, the cuvette carrier may comprise fourth transmissive region, and optionally a fourth optical polariser arranged to polarise light passing through the fourth transmissive region. For example, the fourth transmissive region may provide means for illuminating a region within the holding volume, and the third transmissive region for detecting scattered light from the illuminated region within the holding volume.

In some such embodiments, an axis of polarisation of the fourth optical polariser may be parallel to an axis of polarisation of the second optical polariser.

The fourth transmissive region may be located in the second wall. In one example of use of a cuvette carrier according to this embodiment, the second and fourth transmissive regions may be used as windows for light entering a cuvette. The first and third polarisers may be used to polarise back-scattered light resulting from the interaction of a light beam with a sample in the cuvette carrier, the light beam having entered the cuvette carrier through the second and fourth transmissive regions respectively. In particular, the polarisation axes of the first and third polarisers may be aligned orthogonally to each other, for example so that one polariser of the cuvette carrier may transmit only the horizontal component of scattered light, and another polariser may transmit only the vertical component of scattered light. The polarisation of light entering the cuvette carrier via the second and fourth transmissive regions may be controlled by the inclusion of the second and fourth optical polarisers, aligned to have parallel axes of polarisation so that the sample is always illuminated with light of the same polarisation.

By rotating the cuvette carrier, both components of the scattered light may be detected, and so a full DDLS measurement may be performed in a retrofitted system.

In some embodiments, the first wall may be adjacent to the second wall. This may be the case, for example, when the back-scattered light is to be measured as discussed above. Alternatively, the first wall may be opposite the second wall. This set-up may be used for example to detect the polarisation of forward scattered light: light passing through the second transmissive region and forward scattered by the sample may pass through and be polarised by the third optical polariser, and light passing through the fourth transmissive region and forward scattered by the sample may pass through and be polarised by the first optical polariser. The polarised light may then be detected by a detector, for example a detector in a DLS instrument.

In some embodiments, the fourth transmissive region may be located in a third wall, wherein the third wall is opposite to the second wall. This set-up may be used, for example to detect the polarisation of side-scattered light. For example, light passing through the second transmissive region and side-scattered by the sample may pass through and be polarised by the third optical polariser, and light passing through the fourth transmissive region and side-scattered by the sample may pass through and be polarised by the first optical polariser. The polarised light may then be detected by a detector, for example a detector in a DLS instrument.

In some embodiments, each transmissive region may be located in a different wall of the plurality of walls. For example, the first transmissive region may be opposite to the third transmissive region, and the second transmissive region opposite to the fourth transmissive region.

In some embodiments, at least one of the first, second, third, or fourth transmissive regions may comprise an aperture in a wall. Alternatively, any of the transmissive regions may comprise an optical component, for example a window or lens. At least one of the first, second, third, or fourth optical polarisers may be arranged to polarise all of the light passing through the first, second, third or fourth transmissive region respectively.

The first and second transmissive regions may be adjacent to each other so as to form a continuous transmissive region. For example, the first and second transmissive regions may comprise separate parts of a single aperture in the first wall. Similarly, the third and fourth transmissive regions may be adjacent to each other so as to form a continuous transmissive region.

The optical polariser or optical polarisers may be configured to transmit light from the ultraviolet, visible, or near infrared regions of the electromagnetic spectrum, for example light with a wavelength in the range 280-2000 nm.

In some embodiments, the plurality of walls may define a substantially cuboidal holding volume. The holding volume may be adapted to receive a standard size cuvette, for example a 10 mm×10 mm sample cuvette. The cuvette carrier may further comprise an insert adapted to removably fit into the holding volume, the insert comprising a plurality of walls defining an inner volume that is smaller than the holding volume. For example, inner dimensions of the inner volume of the insert may be approximately 3 mm×3 mm. The insert may allow the cuvette carrier to be compatible with different sizes of cuvettes or smaller glass capillaries. For example, large cuvettes may fit into the holding volume without the insert being inserted into the holding volume, whereas smaller cuvettes may require the insert to be inserted into the holding volume, the smaller cuvettes being held by the insert.

The exterior shape of the cuvette carrier may be substantially cuboidal. An external horizontal cross-region of the cuvette carrier may have dimensions of approximately 12.5 mm×12.5 mm. These dimensions match the dimensions of a standard large cuvette, and so a cuvette carrier of these dimensions may fit into the cuvette holder of an existing DLS system, allowing easy retrofitting of the DLS system.

The plurality of walls may comprise a metal, for example aluminium or stainless steel. The plurality of walls may be substantially covered in a protective coating. For example, if the plurality of walls comprise aluminium, and the protective coating may be an anodised layer. A metal construction aids thermal contact with the cell area of an existing DLS system.

In some embodiments, the cuvette carrier may further comprise a narrow band filter arranged to filter light passing through the first and/or third transmissive regions. In particular, the narrow band filter may be adapted to allow transmission of light at the same wavelength as the illuminating light source for the DLS measurement. The narrow band filter may for example allow transmission of light within a range of ±1 nm of the wavelength of the illuminating light source. This may be useful for excluding fluorescent light, which may otherwise introduce noise into a DLS measurement.

In some embodiments, at least one of the optical polarisers may be configured to be removable by a user. For example a user may replace an optical polariser with a different optical polariser, or may rotate an optical polariser in order to rotate the axis of polarisation of the polariser.

According to a second aspect of the invention there is provided an apparatus for measuring particle size distribution of a sample by light scattering comprising: a light source for generating a beam of light; a cuvette according to any embodiment of the first aspect of the invention; and a detector for detecting scattered light.

According to a third aspect of the invention there is provided a method of testing the optical properties of a liquid sample, the method comprising: placing the sample in a cuvette; placing the cuvette in a cuvette carrier, the cuvette carrier comprising a plurality of walls defining a holding volume for a cuvette, a first and second transmissive region included in the plurality of walls, and a first optical polariser arranged to polarise light passing through the first transmissive region; illuminating the sample with a test beam from a light source, the test beam passing through the second transmissive region of the cuvette carrier; and detecting light scattered from the sample that passes through the first optical polariser.

In some embodiments, light that is back-scattered, side-scattered, or forward-scattered from the sample may be detected.

The method may further comprise replacing the first optical polariser with a replacement optical polariser, the replacement optical polariser having an axis of polarisation that is orthogonal to the axis of polarisation of the first optical polariser; and detecting light scattered from the sample that passes through the replacement optical polariser.

Alternatively, the cuvette carrier may further comprise a third transmissive region, a fourth transmissive region, and third optical polariser arranged to polarise light passing through the third transmissive region, the method further comprising; rotating the cuvette carrier such that the test beam passes through the fourth transmissive region of the cuvette carrier; and detecting light scattered from the sample that passes through the third optical polariser. In this way both horizontal and vertical components of scattered light may be detected, and used for depolarised dynamic light scattering analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a cuvette carrier according to the first aspect of the invention, with a cuvette for placing into the cuvette carrier;

FIG. 2A is a schematic representation of an embodiment of a cuvette carrier;

FIG. 2B is a schematic representation of the same embodiment of cuvette carrier as in FIG. 2A, rotated by 90° compared to FIG. 2A;

FIG. 3 is a schematic representation of an alternative cuvette carrier;

FIG. 4 is a top-down view of an embodiment of a cuvette carrier for measuring side scattered light; and

FIG. 5 is a top-down view an alternative embodiment of a cuvette carrier for measuring side scattered light;

FIG. 6 is a schematic representation of an alternative cuvette carrier;

FIGS. 7A to 7D show experimental results obtained with and without a vertical polariser in place between scattered light and a detector used to perform a DLS measurement; and

FIGS. 8A to 8D show experimentally obtained intensity weighted size distributions obtained for samples comprising a 1:3 mixture or a 1:10 mixture of 60 nm and 200 nm latex particles.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary embodiment of a cuvette carrier 100 according to the first aspect of the invention. Cuvette carrier 100 comprises a plurality of walls 101 a-d that define a holding volume 102. In the illustrated embodiment, cuvette carrier 100 comprises four walls of equal width, arranged to form a holding volume 102 with a square shaped cross-section. However, in other embodiments, other numbers of walls are possible, forming otherwise shaped holding volumes. For example, cuvette carrier 100 may comprise 1, 2, 3, 5, or 6 walls. The walls 101 may be of equal widths, or may be of different widths.

A first wall 101 a of the plurality of walls comprises a first transmissive region 103 (shown by a dotted line in FIG. 1) and a second transmissive region 104. A first polarising filter 105 entirely covers the first transmissive region 103, so that any light passing through the first transmissive region 103 passes through the first polarising filter 105. In this embodiment, the second transmissive region 104 is an aperture in the wall 101 a, allowing light to pass into the holding volume 102. In the illustrated embodiment, the first transmissive region 103 and the second transmissive region 104 form a single aperture through the first wall 101 a, but in other embodiments these regions may be separated.

A second wall 101 b of the plurality of walls comprises a third transmissive region 106 (shown by a dotted line in FIG. 1) and a fourth transmissive region 107. A third polarising filter 108 entirely covers the third transmissive region 106, so that any light passing through the third transmissive region 106 passes through the third polarising filter 108. In the illustrated embodiment, the third and fourth transmissive regions 106, 107 form a single aperture in the second wall 101 b, similar to the first and second transmissive regions 103, 104 in the first wall 101 a.

A third wall 101 c of the plurality of walls may comprise a fifth transmissive region 111 (not shown in FIG. 1, but visible in FIG. 2B) for allowing a light beam entering the cuvette carrier 100 through the second transmissive region 104 to exit the cuvette carrier 100 (for instance to avoid reflections of the light beam from contributing to optical noise in a measurement of scattered light). Similarly, a fourth wall 101 d of the plurality of walls may comprise a sixth transmissive region (not shown), for allowing a light beam entering the cuvette carrier 100 through the fourth transmissive region 107 to exit the cuvette carrier 100

It is not essential for a fifth transmissive region 111 to be provided opposite the second transmissive region 104, or for a sixth transmissive region to be provided opposite the fourth transmissive region. In an alternative arrangement a reflector element may instead be provided within the cuvette carrier 100, arranged to cause a light beam entering the cuvette carrier 100 via the second transmissive region to exit the cuvette carrier via the second transmissive region (or the fourth transmissive region, or any other suitable transmissive region), and a similar reflector may be provided, arranged to cause a light beam entering the cuvette carrier 100 via the fourth transmissive region to exit the cuvette carrier via the fourth transmissive region (or the second transmissive region, or any other suitable transmissive region). Other embodiments may be arranged so that a light beam entering via the second or fourth transmissive region may respectively exit the cuvette carrier via the fourth or second transmissive region.

The cuvette carrier 100 may be used to investigate the depolarised dynamic light scattering (DDLS) properties of a sample in a cuvette 109. A cuvette 109 may be placed into the holding volume 102. The cuvette carrier 100 may then be placed in a DLS instrument, such that a beam from a light source illuminates the sample in the cuvette 109. For example, the cuvette carrier 100 may be placed in a cuvette carrier of an existing DLS system so as to retrofit the DLS system for DDLS measurements.

FIGS. 2A and 2B illustrate one way in which the cuvette carrier shown in FIG. 1 may be used for DDLS measurements. In this embodiment, the first polarising filter 105 is aligned so that it polarises light orthogonally to the direction of polarisation of light passing through the third polarising filter 108. For example, the first polarising filter 105 may polarise light vertically, and the third polarising filter 108 may polarise light horizontally. These directions are represented by the arrows on the scattered light beams 202 in FIGS. 2A and 2B.

FIG. 2A shows a first step of the measurements. Incoming light 201 from an external light source is used to illuminate the sample in the cuvette carrier. The light source may be, for example, a laser, for example a laser that is part of an existing DLS experimental system. The light source may provide light with a wavelength that is in the range of 280 to 2000 nm.

Incoming light 201 passes through the second transmissive region 104, and enters the holding volume 102. Particles in sample held in the holding volume 102 may scatter the light. The light may be scattered in any direction, however in the particular embodiment illustrated only back-scatted light is measured—e.g. light that is scattered within a range of 160°-200° from the direction of the incoming light 201. The back-scattered light 202 passes through the first transmissive region 103, and is polarised by the first optical polariser 105. Substantially only the component of scattered light 202 that is polarised in the same direction of the axis of polarisation of the first optical polariser 105 will pass through the optical polariser 105. In the illustrated embodiment, only the vertically polarised component of the back-scattered light 202 passes through the polariser 105, and may be detected by a detector. The detector may, for example, be a photodiode, and may be the detector of an existing DLS instrument.

The incoming light 201 passing through the second transmissive region 104 may exit the holding volume 102 via the fifth transmissive region 111.

The horizontally polarised component of the scattered light may be measured. This is shown in FIG. 2B. The cuvette carrier 100 may be rotated by 90° so that the second wall 101 b faces the illuminating light 201. This may be achieved manually, for example by removing the cuvette carrier 100 from a cuvette holder of a DLS system, rotating the cuvette carrier 100 by 90°, and replacing the cuvette carrier 100 in the cuvette holder. Alternatively a motor may be used to automatically rotate the cuvette carrier 100. Alternatively the cuvette carrier 100 may be held fixed, but the direction of the incoming light 201 rotated by 90° so that the second wall 101 b is illuminated.

In FIG. 2B, the incoming light 201 passes through the fourth transmissive region 107, and is scattered by the sample in the holding volume 104. Again, in the illustrated embodiment only back-scattered light is measured. Back-scattered light 202 passes through the third transmissive region 106, and is polarised by the third optical polariser 108, so that only the horizontally polarised component of the back-scattered light 202 passes through the polariser 108. This light may then be detected by a detector.

The incoming light 201 passing through the fourth transmissive region 107 may exit the holding volume 102 via the sixth transmissive region (not shown).

In this way, both the vertical and horizontal components of scattered light may be measured. Combining these measurements, the polarisation of the back-scattered light may be calculated, and used for DDLS calculations.

In some circumstances, it may be important to ensure that the illuminating light has a known polarisation state. This may particularly be the case if the light source is not a laser. FIG. 3 shows an alternative embodiment of a cuvette carrier 300 that may be used in such cases.

The cuvette carrier 300 comprises all of the features of cuvette carrier 100. Additionally, a second optical polariser 301 covers the second transmissive region 104, and a fourth optical polariser 302 covers the fourth transmissive region 107. The second 301 and fourth 302 optical polarisers may in particular be aligned so that they polarise light along the same axis—for example both polarisers 301, 302 may polarise light horizontally, or both polarisers 301, 302 may polarise light vertically.

Cuvette carrier 300 may be used for DDLS measurements of back-scattered light in the same way as described for cuvette carrier 100. In this case, incoming light 201 entering the second 104 or fourth 107 transmissive regions will pass through the second 301 and fourth 302 optical polarisers respectively, before entering the sample. The incoming light will therefore be polarised along a known direction.

The examples above have described the measurement of back-scattered light only. However, a skilled person will readily understand that the cuvette carrier may be adapted to detect light scattered at any angle, using a similar method to the measurements described above. For example, any of the first, second, third, and fourth transmissive regions, and their respective optical polarisers, may be located on any of the plurality of walls of the cuvette carrier, in order to detect any direction of scattered light. In particular, the first optical polariser on the first transmissive region may be used to polarise scattered light resulting from the interaction with the sample of a light beam passing through the second transmissive region, and the third optical polariser on the third transmissive region may be used to polarise scattered light resulting from the interaction with the sample of a light beam passing through the fourth transmissive region. In any embodiment, the second and fourth transmissive regions may be covered by second and fourth optical polarisers, in order to ensure a known, and common, polarisation of light entering the sample, as described above.

For example, FIG. 4 shows an embodiment of a cuvette carrier 400 that may be used to measure side-scattered light, i.e. light that is scattered by substantially 70°-410° from the direction of incoming light. In cuvette carrier 400, each transmissive section 401-404 is located on a different wall of the plurality of walls 405 a-d. The first transmissive section 401 is located on a first wall 405 a. The second transmissive section 402 is located on a second wall 405 b that is adjacent to the first wall 405 a. The third transmissive section 403 is located on a third wall 405 c that is opposite to the first wall 405 a. The fourth transmissive section is located on a fourth wall 405 d that is opposite to the second wall 405 b. The second 402 and fourth 404 transmissive sections may or may not be covered by second and fourth optical polarisers.

DDLS measurements using the cuvette carrier 400 may proceed in a similar way to the method used for cuvette carrier 100. The polarisation axis of the first 406 and third 407 optical polarisers are aligned orthogonally to one another, for example the first optical polariser 406 may be aligned to transmit only vertically polarised light, and the third optical polariser may be aligned to transmit only horizontally polarised light (denoted in the figure by the V and H respectively on the scattered beams 409 a and 409 b).

In a first step, vertically polarised side-scattered light is measured. An incoming light beam 408 passes through the second transmissive region 402 and is scattered by a sample in the cuvette carrier 400. Side-scattered light 409 a passes through the first transmissive region 401. The vertically polarised component of this side-scattered light 409 a passes through the first optical polariser 406, and is detected by a detector. The incoming light beam 408 passing through the second transmissive region 402 may exit the cuvette carrier 100 via the fourth transmissive region 404.

In a second step, horizontally polarised side-scattered light is measured. The cuvette carrier is rotated 180°, so that the incoming light beam 408 passes through the fourth transmissive region 404 and is scattered by a sample in the cuvette carrier 400. Side-scattered light 409 b passes through the third transmissive region 403. The incoming light beam 408 passing through the fourth transmissive region 404 may exit the cuvette carrier 100 via the second transmissive region 402. The horizontally polarised component of this side-scattered light 409 b passes through the third optical polariser 407, and is detected by a detector. Thus both the vertical and horizontal components of side-scattered light are measured, and used for DDLS analysis.

FIG. 5 illustrates an alternative method of using a cuvette carrier 500 to measure side-scattered light. Cuvette carrier 500 comprises a first 501, second 502, third 503 and fourth 504 transmissive region, located in a first 504 a, second 504 b, third 505 c and fourth 505 d wall of the plurality of walls respectively. In this embodiment for measuring side-scatted light, the first wall 504 a is adjacent to the second wall 504 b, and the third wall 504 c is adjacent to the second wall 504 b and opposite to the first wall 504 a. The fourth wall 505 d is opposite to the second wall 504 b.

Each transmissive section 501, 502, 503 is covered by a respective optical polariser 505, 506, 507. The fourth transmissive region 504 is not covered by an optical polariser. The first optical polariser 505 and second optical polariser 506 are aligned to polarise light along the same axis—for example vertically. The third optical polariser 507 is aligned to polarise light along an axis that is orthogonal to the axis of the first 505 and second 506 polarisers—for example horizontally.

For DDLS measurements, the cuvette carrier 500 is first arranged so that incoming light 508 passes through the first optical polariser 505 and first transmissive region 501, ensuring that the light entering the sample is vertically polarised, in this example. The light is then side-scattered by the particles in the sample. The incoming light 508 entering the cuvette via the first transmissive region 501 that is not scattered from the sample exits the cuvette carrier 500 via the third transmissive region 503. Side-scattered light 509 a passes through the second transmissive region 502, and is polarised by the second polariser 506 so that only vertically polarised side-scattered light 509 a passes through the second polariser and is detected by a detector.

To measure the horizontal component of side-scattered light, the cuvette carrier 500 may be rotated by 90°, so that incoming light 508 now enters the sample through the second transmissive region 502. The second optical polariser 502 will ensure that only vertically polarised incoming light 508 is transmitted into the sample. The incoming light 508 entering the cuvette via the second transmissive region 502 that is not scattered from the sample exits the cuvette carrier 500 via the fourth transmissive region 504. Light that is side-scattered by the sample then passes through the third transmissive region 503. The third optical polariser 507 only allows the horizontal component of the side-scattered light 509 b to be transmitted, which is then detected by a detector. Thus the vertical and horizontal components of side-scattered light may be measured using only three transmissive regions.

FIG. 6 shows an alternative embodiment of a cuvette carrier 600. The cuvette carrier 600 comprises some or all of the features of cuvette carrier 100, which are given like reference numerals where applicable. For example, a first wall 101 a of the plurality of walls comprises a first transmissive region 103 (shown by a dotted line in FIG. 6) and a second transmissive region 104. A first optical polarizer 105, e.g. polarising filter 105, entirely covers the first transmissive region 103, so that any light passing through the first transmissive region 103 passes through the first polarising filter 105. In this embodiment, the second transmissive region 104 is an aperture in the wall 101 a, allowing light from an illuminating source for DLS to pass into the holding volume 102. In the illustrated embodiment, the first transmissive region 103 and the second transmissive region 104 form a single aperture through the first wall 101 a, but in other embodiments these regions may be separated by a portion of the wall, i.e. they may be separate apertures.

Additionally, a second optical polariser 601, e.g. polarising filter 601, covers the second transmissive region 104. In embodiments, the axis of polarisation of the first optical polariser 105 is parallel to the axis of polarisation of the second optical polariser 601. In embodiments, the axis of polarisation of the first optical polariser 105 is vertical and the axis of polarisation of the second optical polariser 601 is also vertical. Vertical polarisation is to be understood in the context of a DLS measurement, wherein the scattering plane, i.e. the plane containing the q-vector, is orthogonal to the polarisation of the illuminating laser. In embodiments, the axis of polarisation of the first optical polariser and the axis of polarisation of the second optical polariser are both parallel to each other and also parallel to the polarisation of an illuminating laser. In embodiments they may all be vertically polarised. It should be appreciated that if the illuminating laser used in the DLS measurement has a high polarisation extinction ratio (e.g. greater than 3000:1) then it may not be necessary to have a vertical polariser, e.g. second optical polariser 601, in the beam path between the illuminating laser and a sample on which a DLS measurement is to be performed. In this case, the illuminating laser may be polarised parallel to the first optical polariser 105 which is in the path between scattered light and a detector used to perform a DLS measurement, e.g. both may be vertically polarised.

Cuvette carrier 600 may be used for DLS measurements based on back-scattered light from a sample within a cuvette. In this case, incoming illuminating light 201 enters through the second 104 transmissive region and will therefore pass through the second optical polariser 601 before entering the sample. The sample will therefore illuminated by light polarised along a known direction, e.g. vertically. The back-scattered light 202 from the sample passes through the first transmissive region 103, and is polarised by the first optical polariser 105, e.g. vertically, before reaching a detector for taking DLS measurements. Whilst the cuvette carrier 600 has been illustrated with additional transmissive regions and polarisers in a second wall 101 b, it is to be appreciated that these are not essential elements of the embodiment, e.g. for a backscattering measurement it may only be necessary to have first 103 and second 104 transmissive regions in a single wall 101 a.

It has been appreciated that stress-birefringence is locked into many plastic mouldings, such as plastic cuvettes, as the molten plastic often cools quickly in the tool, due to injection profiles optimised for fast cooling and thence increased throughput. DLS generally relies upon the existence of a well-defined ‘scattering plane’. For cuvettes that scramble the direction of polarisation of the light, but leave it linearly polarised, an uncertainty is caused in the overall measured intensity at a specific but fixed detection angle from the sample. However, as long as this polarisation remains stable then the dynamics of the measurement are not affected. An issue appreciated by the present inventor is that random scrambling of the polarisation is more likely to occur, resulting in elliptically polarised light in the detection volume. This means that a z-dependent scattering plane exists and any size-based structure in the scattered field will be detected as a variation in the measured intensity along a specific and fixed detection angle. If this change in the detected light level occurs on the same timescales as the diffusional processes within the sample, then an uncertainty in the measured dynamics of the scattered light results, with the potential to directly affect the accuracy and precision of DLS measurement. Stress features in the plastic of the cuvette that are micro- or nano-scaled will also scatter light themselves. That is, scattering from birefringent regions, or between domains of differing birefringence in the cuvette wall, will take part in the measurement but only to add static scattering (noise) to the signal. Static scattering is uncorrelated and therefore this process would work to reduce the intercept of the measured autocorrelation function, which is commonly interpreted as associated with the signal to noise in the measurement. It has been appreciated that cuvette carrier 600 can ameliorate at least some of the potential issues resulting from using plastic cuvettes, which are otherwise desirable for their low cost and disposability.

FIGS. 7A to 7C show the results of comparative experimental DLS measurements of dilute 200 nm polystyrene latex particles dispersed in 150 mM sodium chloride, prepared in 200 nm filtered deionized water (i.e. the filter used to prepare the water has pores to remove material larger than 200 nm). The sample was contained within a plastic cuvette. In each figure, the left-hand box plot represents the data obtained using a cuvette carrier 600 comprising a pair of vertical polarisers 601 and 105 covering the first and second transmissive regions, whereas the right-hand box plot represents data obtained using a plastic cuvette carrier without a vertical polariser 105 between the detector and the sample in the backscattered beam 202 (although the illuminating laser is inherently vertically polarized). The results indicate reduced uncertainties associated with the polarised measurement in disposable plastic cuvettes. The Zave (the average reported particle size) shown in FIG. 7A, the PDI (a metric associated with the width of the measured particle size distribution) shown in FIG. 7C and the Intercept (read, signal to noise) shown in FIG. 7B are all superior to the data recorded without the output polariser in place. It should be appreciated that the Intercept is better when closer to 1, and PDI in theory could be zero, but in reality is a finite value greater than zero due to a convolution of optical noise, sample dispersity and instrument factors. The results show that PDI values closer to zero may be measured with the polarisers in place, and hence may allow a more sensitive characterisation of changes in samples.

FIG. 7D shows the results of an experiment, similar in all respects to that related to FIGS. 7A to 7C, but for a 60 nm polystyrene latex particles instead of 200 nm particles. FIG. 7D shows box and whisker plots representative of experimentally derived Zave for three cases: 1) a plastic cuvette wherein no polariser 105 is placed between the detector and the cuvette in the path 202 of the scattered light 710; 2) a plastic cuvette wherein a vertical polariser 105 is placed between the detector and the cuvette in the path 202 of the backscattered light 720; and 3) a quartz cuvette wherein no polariser is placed between the detector and the cuvette in the path 202 of the scattered light 730. It is evident from the plot that with the inclusion of a vertical polariser 105 in the backscattered light, the results obtained with the plastic cuvette show a similar distribution to the results obtained with the quartz cuvette. In other words, cuvette carrier 600 enables the measurement precision with plastic cuvettes to be similar to the measurement precision typically obtainable with high quality quartz cuvettes, but without the associated disadvantages such as cost.

The inclusion of a vertical polariser 105 in the path of the backscattered light has also been found to improve the resolution of multimodal size distributions, as shown in FIGS. 8A to 8D for DLS measurements performed on a sample comprising a mixture of 60 nm and 200 nm latex particles. DLS is sometimes considered to have a relatively poor resolution in terms of the ability to resolve two separate sample populations present in the same aliquot. FIGS. 8A and 8B show experimentally obtained intensity weighted size distributions obtained for a sample comprising a 1:3 mixture of 60 nm and 200 nm latex particles. For the measurement of FIG. 8A all output polarisations were detected in the backscattered light, i.e. no vertical output polariser 105 was used. For the measurement of FIG. 8B a vertical polariser 105 was used in the backscattered light, with its axis parallel to the second optical polariser 601, i.e. both vertical. The results indicate that whilst the 1:3 mixture could be resolved in both cases, they were significantly more reproducible when measured with the polarisers in place. FIGS. 8C and 8D show corresponding measurements to FIGS. 8A and 8B respectively except for a 1:10 mixture of 60 nm and 200 nm latex particles in the sample. It is noted that the 1:10 ratio could not be resolved as bimodal without the first polariser 105 with its axis parallel to the second polariser 601 (i.e. both vertical). Similar results are anticipated if instead of using the second polariser 601 an input vertically polarised illuminating laser is used, parallel to the first polariser 105 ion the path of the backscattered light between the sample and the detector.

The use of a Vin-Vout polariser (i.e. combination of first polariser 105 and second polariser 601, both vertically polarised) mounted on a cuvette holder, or a Vout polariser (i.e. first polariser 105) mounted on the cuvette-holder and with the sample illuminated with a laser of high polarisation extinction ratio (>3000:1, for example), it has been demonstrated that it is possible to retrieve the high quality, glass cell ZAve and PDI performance from a disposable plastic cell. The polarisers were able to improve the resolution of the DLS measurement to a 1:10 split of 60 nm and 200 nm data (from 1:3 resolved without polariser).

It is to be understood that the above embodiments are only examples of cuvette carriers according to this invention, and that many other embodiments of cuvette carriers, and methods of using them for DDLS measurements, are possible within the scope of the invention. In particular, the arrangement of transmissive regions may be chosen to measure light that is scattered at any angle, including back-scattered, side-scattered, and forward-scattered light. A cuvette carrier may comprise more than four transmissive regions, and respective optical polarisers, to enable a single cuvette carrier to be used to measure different directions of scattered light. The optical polarisers may be removable or changeable by the user. For example the second and fourth polarisers may be removable, or any orientation of any of the polarisers may be changed to rotate their polarisation axis.

Additionally, any embodiment may further comprise at least one narrow band filter covering at least one of the transmissive regions, and in particular transmissive regions that are in the path of the scattered light to be detected. The narrow band filter may allow transmission of light with a wavelength in the range of ±1 nm of the wavelength of the illuminating light source. The narrow band filter may be required in cases where the sample fluoresces under illumination by the light source. Fluorescent light is uncorrelated, and therefore degrades the DLS measurement. The narrow band filter may substantially remove the fluorescent light from the scattered light beam. The narrow band filters may be removable by the user, so that unnecessary loss due to the filters is avoided when the sample does not fluoresce at the illumination wavelength.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims. 

The invention claimed is:
 1. A cuvette carrier for use in an apparatus for measuring particle size distribution of a sample by dynamic light scattering, the cuvette carrier comprising: a plurality of walls defining a holding volume for a cuvette; a first and second transmissive region included in the plurality of walls; a first optical polariser arranged to polarise light passing through the first transmissive region; and a second optical polariser arranged to polarise light passing through the second transmissive region, wherein an axis of polarisation of the first optical polariser is parallel to an axis of polarisation of the second optical polariser.
 2. The cuvette carrier of claim 1, wherein the axis of polarization of the first optical polarizer is vertical and the axis of polarization of the second optical polarizer is also vertical.
 3. The cuvette carrier of claim 1, wherein the first transmissive region and the second transmissive region are both located in a first wall of the plurality of walls, such that one of the first or second transmissive regions is arranged to receive light which has been backscattered from a sample when in use.
 4. The cuvette carrier of claim 1, wherein the optical polariser or optical polarisers are configured to transmit light with a wavelength in the range 280-2000 nm.
 5. The cuvette carrier of claim 1, further comprising a narrow band filter arranged to filter light passing through the first and/or second transmissive regions.
 6. An apparatus for measuring particle size distribution of a sample by dynamic light scattering, the apparatus comprising: a light source for generating a beam of light; a cuvette carrier according to claim 1; a cuvette placed in the holding volume of the cuvette carrier; and a detector for detecting scattered light which arises from scattering of the beam of light from a sample contained within the cuvette.
 7. The apparatus according to claim 6, wherein the scattered light arises due to backscattering of the beam of light from the sample.
 8. The apparatus according to claim 6, wherein the polarization of the beam of light is parallel to the axis of polarisation of the first optical polariser and parallel to the axis of polarisation of the second optical polariser.
 9. The apparatus according to claim 8, wherein the polarization of the beam of light is vertical.
 10. The apparatus according to claim 6, arranged such that the beam of light is arranged to enter the holding volume of the cuvette carrier through the second transmissive region.
 11. The apparatus according to claim 6, wherein the detector is arranged to detect scattered light that passes from the cuvette through the first transmissive region towards the detector.
 12. The apparatus according to claim 6, wherein the cuvette is formed of plastic.
 13. An apparatus for measuring particle size distribution of a sample by dynamic light scattering, the apparatus comprising: a light source for generating a beam of light; a cuvette carrier comprising: a plurality of walls defining a holding volume for a cuvette; a first and second transmissive region included in the plurality of walls; a first optical polariser arranged to polarise light passing through the first transmissive region; a cuvette placed in the holding volume of the cuvette carrier; and a detector arranged to detect scattered light which arises from scattering of the beam of light from a sample contained within the cuvette and which passes through the first transmissive region towards the detector, wherein an axis of polarisation of the first optical polariser is parallel to an axis of polarisation of the beam of light.
 14. The apparatus according to claim 13, wherein the axis of polarization of the first optical polariser is vertical and the axis of polarization of the beam of light is vertical.
 15. The apparatus according to claim 13, wherein the scattered light arises due to backscattering of the beam of light from a sample contained within the cuvette.
 16. The apparatus according to claim 13, wherein the beam of light is arranged to enter the holding volume of the cuvette carrier through the second transmissive region.
 17. The apparatus according to claim 13, further comprising a second optical polariser arranged to polarise light passing through the second transmissive region, wherein an axis of polarisation of the second optical polariser is vertical.
 18. The apparatus according to claim 13, wherein the cuvette is formed of plastic.
 19. A method of characterising particles dispersed in a sample, the method comprising: placing the sample in a cuvette; placing the cuvette in a cuvette carrier, the cuvette carrier comprising: a plurality of walls defining a holding volume for a cuvette; a first and second transmissive region included in the plurality of walls; a first optical polariser arranged to polarise light passing through the first transmissive region; a second optical polariser arranged to polarise light passing through the second transmissive region, wherein an axis of polarisation of the first optical polariser is parallel to an axis of polarisation of the second optical polariser; illuminating the sample with a light beam from a light source, the light beam passing through the second transmissive region of the cuvette carrier; and detecting scattered light from the sample produced by the interaction of the light beam with the particles, the scattered light passing through the first transmissive region.
 20. The method of claim 19, wherein the axis of polarization of the first optical polarizer is vertical and the axis of polarization of the second optical polarizer is vertical. 